1. Field of the Invention
The invention relates to a rapid thermal process method and a rapid thermal process device for practicing the rapid thermal process method, and more particularly, to a rapid thermal process method and a rapid thermal process device containing a pre-heating process by means of a laser beam.
2. Description of the Prior Art
With the advancing technology of the semiconductor industry, integrated circuits (ICs) are being developed to increase the current computing and storage capability, which pushes the development of related manufacturers forward. As predicted by Moore's law, the number of transistors doubles every 18 months. The process of semiconductor evolves from 0.18 μm of 1999, 0.13 μm of 2001, 90 nm of 2003 to 65 nm of 2005 and is approaching 45 nm. Therefore, the density of semiconductor elements on a wafer is increasing with the technology advancement of the semiconductor industry and miniaturization of microelectronic elements and makes the intervals between elements shorter and shorter. Under this situation, many semiconductor fabrication processes face new challenges and bottlenecks, and therefore the manufacturers have to keep on researching new fabrication technologies to meet the request of high integration.
Among various semiconductor fabrication processes, the rapid thermal process (RTP) is a very important technology and has been widely applied to the thermal activating of semiconductor processes in the fabrication of very large scale integration (VLSI) field. Its application may contain the formation of the ultra shallow junction (USJ) of metal-oxide-semiconductor (MOS) transistors, ultra thin oxide layer growth, annealing, diffusion, formation of metal silicide, and even the semiconductor layer of thin film transistors. With the advancing technology of the semiconductor industry, rapid thermal processes are being developed to meet the requirements of high fabrication grades. According to the development of thermal processes, high-temperature furnace is a representative tool in earlier technology, and the spike rapid thermal annealing is utilized for rapid thermal treatment in the 90 nm grade process. Currently, as the semiconductor technology is developed to the 65 nm grad process, new rapid thermal processes, such as flash/non-melt annealing, impulse and laser annealing, are researched to be applied. Correspondingly, the process time of a thermal process becomes shorter and shorter. For example, the process time is about 10 sec for the earlier furnace process, and the process time is shortened to about 1 sec, even about 1 msec (millisecond) for the current thermal process.
The laser thermal process is applied to rapidly heating and cooling semiconductor materials, such as the application of laser annealing process, and has been widely applied to the fabrication of display devices, such as liquid crystal displays (LCD). In the liquid crystal display devices, since a normal glass substrate can only work at a temperature below 600° C., fabricating a polysilicon film directly under a high temperature will make the glass substrate twisted. Thus, in a conventional method of fabricating a polysilicon thin film transistor, an expensive quartz substrate is needed and only a small size liquid crystal display panel can be made. Recently, a method of forming a low temperature polysilicon (LTPS) thin film transistor (TFT) by crystallizing an amorphous silicon film is gradually developed for the mass production of large size liquid crystal displays. Among the methods of forming polysilicon film, the excimer laser annealing (ELA) process is the major focus, which means the excimer laser is used as a heat source to transfer the amorphous silicon structure into a polysilicon structure. When the excimer laser passes through an optical projection system, a laser beam with uniform energy is generated that projects to the substrate having the amorphous film thereon so that the amorphous film may absorb the energy and transfer into a polysilicon film. Since the above-mentioned process can be finished below 600° C., it can be applied to general glass substrates or plastic substrates. Therefore, the application field of LTPS TFT liquid crystal displays is broadened.
As a result, the laser thermal process has become a very important process technology whether in the fabrication process of the IC field or the display field. According to the prior-art laser thermal process, the substrate (such as a semiconductor wafer) needed a thermal treatment is set on a chuck in a chamber. The chuck with a heater is used to heat the whole substrate to a critical temperature of the substrate, such as 400° C. so that free carriers are generated. Then, the laser beam is used to scan the substrate for performing a rapid heating process to raise the temperature of the substrate to about 1200° C. or higher for completing the annealing or re-crystallization process. However, the design of the chuck is limited so that it is easy to occur temperature non-uniformity on the substrate. In addition, it takes about 3 to 10 minutes for the laser scan at a time, thus the whole substrate has to be heated at the high temperature of 400° C. for 3 to 10 minutes, which easily causes systematic sheet resistance due to temperature non-uniformity and high thermal budget. Even more, the substrate heated by the hot chuck may be broken resulting from the thermal stress. Furthermore, in order to improve the wafer temperature uniformity, tool supplier designs vacuum or electrostatic chucks. However, the vacuum or electrostatic design may still cause wafer breakage. Taking the vacuum chuck as an example, the adsorptive power is unbalanced at various position of the surface of the substrate while the vacuum function is provided, which influences the thermal stress of the surface of the substrate. Therefore, the contact of the laser beam with high temperature easily causes broken of the substrate. As a result, it is difficult to improve the heating performance by modifying the chuck because the modification of the hardware costs much time and money. In addition, the process time of the pre-heating or cooling down of the conventional chuck for a whole substrate is too long to improve the throughout.
Accordingly, how to improve the rapid thermal process with low thermal budget of semiconductor industry and provide a rapid thermal process device that avoids thermal stress is still an important issue of the semiconductor industry.